Electro-chemical measuring sensor

ABSTRACT

An electrochemical measuring sensor for determining the oxygen content of gases, such as exhaust gases of internal combustion engines, includes a housing having an annular sealing seat defined on an inner surface thereof; a sensor element which is comprised of a solid electrolyte body which is oxygen-ion conducting, which has a form of a tube having a closed end and which has an annular shoulder provided thereon; and a seal ring positioned between the annular sealing seat of the housing and the annular shoulder of the solid electrolyte body, and including a plurality of resilient elements distributed thereon. Each of the plurality of resilient elements has a lower portion and an upper portion angled toward the housing, and the seal ring has a clear width so that each lower portion rests on and is mechanically prestressed against the annular shoulder of the sensor element and each upper portion is mechanically prestressed against the housing, whereby a tight seal of the sensor element to the housing is provided and the sensor element is maintained within the housing under mechanical prestress and secure against relative rotation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is based on an electro-chemical measuring sensor fordetermining the oxygen content of gases, in particular for determiningthe oxygen content of exhaust gases of internal combustion engines, witha sensor element inserted into a housing with a seal ring, which has anoxygen ion-conducting solid electrolyte body in the form of a tubeclosed on one side. Electro-chemical measuring sensors are designed, forexample, with a sensor element of a so-called finger shape, wherein asolid electrolyte body in the shape of a closed tube is sealingly fixedin place in a metallic housing. Differentiations are made in connectionwith finger sensors between potential-free and potential-dependentmeasuring sensors. With potential-dependent measuring sensors, the stripconductors of the outer electrodes are brought into contact with thehousing by means of an electrically conductive sealing ring. Withpotential-free measuring sensors, each electrode connection is directlysupplied to a control instrument, so that electrical contact with thehousing is not permitted. Sealing between the solid electrolyte body andthe housing must be provided in either case.

2. Description of the Related Art

Potential-free measuring sensors in particular, wherein a contact foreach electrode connection is disposed on the annular surface of thesolid electrolyte body, require a positionally correct seating of thecontact elements. There is the danger that during the installation ofthe measuring sensor the solid electrolyte body is twisted after havingbeen inserted and that therefore the contacts of the electrodes take upa different position, because of which assured contact is impossible.

A metallic sealing ring, for example, is used for sealing the solidelectrolyte body in the housing, wherein with a potential-free measuringsensor the strip conductor to the outer electrode in the area of themetallic sealing ring must be covered with electrical insulation.

SUMMARY OF THE INVENTION

The measuring sensor in accordance with the invention and having thecharacterizing features that means are provided which maintain thesensor element in the housing under mechanical prestress, which has theadvantage that the sensor element is fixed in place in the housingsecure against relative rotation. By means of this a secure contact ofthe electrode connections is possible, particularly in connection withthe mounting of potential-free measuring sensors.

Advantageous further embodiments of the measuring sensor in accordancewith the invention are possible by the measures recited in the dependentclaims. In particular, the means are constituted by at least oneresilient element formed on the seal ring. The seal ring may have a ringelement on which the resilient elements are formed evenly radiallydistributed. The resilient elements may be embodied with respectivelyone lower part and an outwardly angled off upper part and have a clearwidth (W) in such a way that the lower parts rest against the sensorelement under mechanical prestress and the upper parts are supportedunder mechanical prestress on the housing. The seal ring may consist ofCr--Ni steel with an applied ductile layer of Ni or Cu. A simple fixingin place of the sensor element in the housing, which is secure againstrelative rotation, can be realized by means of a specially designed sealring which maintains the sensor element in the housing under mechanicalprestress. Equipping the seal ring with resilient elements has beenshown to be particularly practical.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is represented in the drawingsand explained in more detail in the following description. Shown are, inFIG. 1, a longitudinal section through the portion of the measuringsensor on the exhaust gas side; in FIG. 2, an enlarged section of thesealing zone X in FIG. 1; in FIG. 3, a top view of a seal ring and, inFIG. 4, a sectional representation along the line IV--IV in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electro-chemical measuring sensor 10 represented in FIG. 1 has ametallic housing 11, which has a thread 12 on its exterior constitutinga fastening means for installation in a gas measuring tube, not shown.The housing 11 has a longitudinal bore 13 with an annular sealing seat14 supporting a metallic seal ring 30. The sensor element 15 rests onthe seal ring 30 with an annular shoulder 17 formed on a bead-shapedhead 16. A sealing surface 18 on the side of the sensor element isformed on the bead-shaped head 16 between the seal ring 30 and thesensor element 15. In the area of the bead-shaped head 16, thelongitudinal bore 13 is designed in such a way that a gap 36 is formedbetween the bead-shaped head 16 of the sensor element 15 and the housing11. The sealing seat 14 itself forms a sealing face 19 on the housingside. The sealing zone X being formed on the seal ring 30 is shown in anenlargement in FIG. 2.

In the instant example the sensor element 15 is a tube-shaped solidelectrolyte body 20, whose end section toward the gas to be measured isclosed. A layered, gas-permeable measuring electrode 21 is disposed onthe exterior exposed to the gas to be measured, and a layered,gas-permeable reference electrode 22, which is exposed to a referencegas, for example air, is disposed on the side facing the interior. Themeasuring electrode 21 is connected by means of a measuring electrodestrip conductor 23 with a first electrode contact 25, and the referenceelectrode 22 is connected by means of a reference electrode stripconductor 24 with a second electrode contact 26. The electrode contacts25, 26 are respectively located on a front face 27 formed by the openend of the solid electrolyte body 20. A porous protective layer 28 isplaced over the measuring electrode 21 and partially over the measuringelectrode strip conductor 23. The electrodes 21, 22 and the stripconductors 23, 24 are advantageously embodied as cermet layers andco-sintered with the solid electrolyte body 20.

The sensor element 15, which protrudes on the gas-measuring side fromthe longitudinal bore 13 of the housing 11, is surrounded at a distanceby a protective tube 40, which has openings 41 for the entry and exit ofthe gas to be measured and is fixed in place on the end of the housingtowards the gas to be measured. The interior of the sensor element isfilled, for example, by a rod-shaped heating element 50 which is fixed,not shown, at the end away from the gas to be measured and is providedwith line connections.

A first contact element 45 rests on the first electrode contact 25 and asecond contact element 46 on the second electrode contact 26. Thecontact elements 45, 46 are shaped in such a way that they rest, forexample, against the tube-shaped heating element 50 and are in contactwith a measuring electrode connector 47 and a reference electrodeconnector 48. The connectors 47, 48 are connected with connectioncables, not shown, and lead outward to a measuring or controlinstrument, also not shown.

An insulating sleeve 52 is furthermore disposed in the longitudinal bore13 of the housing 11 and preferably is made of a ceramic material. Theinsulating sleeve 52 is pressed on the contact element 45, 46 bymechanical means, not shown, and the electrical connection with theelectrode contacts 25, 26 is provided in this way.

The measuring sensor represented in FIG. 1 is a potential-free measuringsensor. Since the metallic seal ring is electrically conducting, themeasuring electrode strip conductor 23 is covered, at least in the areaof the sealing zone X, with an electrically insulating layer 29 (FIG.2).

In place of an insulating layer 29 it is also possible to provide anelectrically insulating layer system consisting of several layers. Inthe instant exemplary embodiment the insulating layer 29 is drawn aroundthe circumference of the solid electrolyte body 20 which adjoins thehousing 11. However, it is also conceivable to limit the insulatinglayer 29 merely to the area of the contact places of the seal ring 30,or to extend the insulating layer 29 on the side toward the gas to bemeasured as far as the protective layer 28. The insulating layer 29 is aplasma-sprayed magnesium-spinel layer.

For fixation in place secure against relative rotation, the sensorelement 15 is maintained under mechanical prestress in the housing 11 bymeans of the seal ring 30. For this purpose the seal ring 30 in FIGS. 3and 4 has a ring element 31, on whose circumference strips 32 of a clearwidth W, which are resilient in the axial direction, are formed andproject into the gap between the bead-shaped head 16 and the housing 11(FIG. 2). In accordance with the instant exemplary embodiment, sixresilient strips 32 are provided which are evenly disposed radially. Theresilient strips 32 respectively have a lower part 33 and an outwardlyangled part 34. The clear width W of the lower parts 33 is selected suchthat the lower parts 33 are seated under prestress on the bead-shapedhead 16. Otherwise the gap 36 is of such size, or the upper parts 34 areoutwardly angled sufficiently far, that the resilient strips 32 aresupported on the interior wall of the longitudinal bore 13. By means ofthis it is assured that the sensor element 15 is held under mechanicalprestress in the longitudinal bore 13 of the housing 11 by means of theseal rings 30 (FIG. 2). The exerted prestressing force maintains thesensor element 15 in the housing 11 so that, up to defined twistingforce, it is fixed in place, secure against relative rotation, in thehousing 11. This fixation in place, secure against relative rotation, isused to keep the sensor element in the radial orientation providedduring assembly.

In the instant exemplary embodiment the seal ring 30 is made of Cr--Nisteel with a ductile layer of Ni or Cu applied to both sides. But it isalso conceivable to employ other materials which permit the productionof correspondingly resiliently embodied strips 32 and in addition assuresufficient sealing. However, the seal ring 30 is not limited toemployment in connection with potential-free measuring sensors.

What is claimed is:
 1. An electrochemical measuring sensor fordetermining the oxygen content of gases including exhaust gases ofinternal combustion engines, the electrochemical measuring sensorcomprising:a housing having an annular sealing seat defined on an innersurface thereof; a sensor element which is comprised of a solidelectrolyte body which is oxygen-ion conducting, which has a form of atube having a closed end and which has an annular shoulder providedthereon; and a seal ring positioned between the annular sealing seat ofthe housing and the annular shoulder of the solid electrolyte body, andincluding a plurality of resilient elements distributed thereon, each ofthe plurality of resilient elements having a lower portion and an upperportion angled toward the housing, and the seal ring having a clearwidth so that each lower portion rests on and is mechanicallyprestressed against the annular shoulder of the sensor element and eachupper portion is mechanically prestressed against the housing, whereby atight seal of the sensor element to the housing is provided and thesensor element is maintained within the housing under mechanicalprestress and secure against relative rotation.
 2. The electrochemicalmeasuring sensor in accordance with claim 1, wherein the seal ringconsists of Cr--Ni steel having a ductile layer of Ni or Cu appliedthereto.
 3. The electrochemical measuring sensor in accordance withclaim 1, wherein the seal ring further comprises a ring element ontowhich the plurality of resilient elements are evenly radiallydistributed thereon.
 4. An electrochemical measuring sensor fordetermining the oxygen content of gases including exhaust gases ofinternal combustion engines, the electrochemical measuring sensorcomprising:a housing having an annular sealing seat defined on an innersurface thereof; a sensor element which is comprised of a solidelectrolyte body which is oxygen-ion conducting, which has a form of atube having a closed end which contacts the gas to be measured and anopen end which is inserted into the housing, and which has an annularshoulder provided near the open end; and a seal ring positioned betweenthe annular sealing seat of the housing and the annular shoulder of thesolid electrolyte body, the seal ring including a ring element and aplurality of resilient elements provided on the ring element and evenlyradially distributed thereon, each of the plurality of resilientelements having a lower portion and an upper portion angled toward thehousing, and the seal ring having a clear width so that each lowerportion rests on and is mechanically prestressed against the annularshoulder of the sensor element and each upper portion is mechanicallyprestressed against the housing, whereby a tight seal of the sensorelement to the housing is provided and the sensor element is maintainedwithin the housing under mechanical prestress and secure againstrelative rotation.
 5. The electrochemical measuring sensor in accordancewith claim 4, wherein the seal ring consists of Cr--Ni steel having aductile layer of Ni or Cu applied thereto.